Hydrogen is an important gas for industrial applications, and as a fuel for fuel cell devices. Any hydrocarbon fuel that contains sulfur containing impurities (e.g. diesel, military strategic fuels, jet fuel, etc.) and that is used to produce hydrogen will convert the sulfur impurities to hydrogen sulfide. Hydrogen sulfide acts as a poison to fuel cell catalysts, and must be removed prior to use in fuel cells or in many industrial applications.
Fuel cells based on solid polymer electrolyte membranes (PEM) have attracted much recent attention due to their promise as energy conversion devices for portable, stationary, and transportation applications. Currently, most PEM fuel cells utilize perfluorosulfonic acid polymer membranes, such as Nafion. This type of membrane is typically operated below 100° C., due mainly to the complex water dynamics within the membrane. Additional technical challenges such as dimensional stability, low mechanical strength, and chemical degradation by peroxide radicals require additional and costly engineering solutions and control strategies. Phosphoric acid doped polybenzimidazoles (PBI) as membranes for high temperature polymer electrolyte membrane fuel cells (PEMFC) show distinct advantages such as high cell operation temperature (between 120° C. and 200° C.), high fuel impurity tolerance, no need for water management, and highly efficient waste heat usage.
Most PEMFCs use hydrogen as the preferred fuel gas and operate on the hydrogen rich gases reformed from other fuel sources. The reformation of other fuels into a hydrogen rich gas will likely contain carbon monoxide (CO) and hydrogen sulfide (H2S) as key fuel gas impurities that could affect the fuel cell performance due to the high sensitivity of platinum catalysts to these fuel impurities. Low temperature PEM systems showed extremely high sensitivity to these fuel impurities, the operational limit of these impurities is at ppm and sub-ppb level for CO and H2S, respectively. Therefore, it is required to remove these fuel impurities from anode feed gas streams, which usually rely on scrubbing technologies. The gas composition of a “clean” stream (from reformation of Navy strategic fuels such as jet fuel and removal of fuel impurities by scrubbing technologies) and the actual concentration levels of the variable components are summarized in Table 1. Similar or higher concentrations of hydrogen sulfide would be expected for high sulfur diesel fuels and other heavy hydrocarbon fuels.
TABLE 1The composition of “clean” gas stream from Navy reformates.Concentration (%) Highest concentration (%) ofGas type(Clean stream)the variable componentsH241.6CH40.1N234.7CO20.0357.7H2O23.5CO10-3 (10 ppm)10H2S5 × 10-8 (5 × 10−4 ppm)2.36 × 10−3 (23.6 ppm)
High temperature PBI-based fuel cells showed excellent tolerance to CO (up to 2%) at typical operational temperatures (i.e., 160° C. to 180° C.), but the fuel cell performance was very sensitive to the CO content at lower operational temperatures (i.e., 140° C. to 120° C.). However, PBI-based fuel cells showed good tolerance to 0.5 ppm H2S and limited tolerance to 25 ppm H2S at temperatures from 120° C. to 180° C.
A hydrogen purification technique, termed electrochemical hydrogen pumping, was developed, originally in the 1960s based on low-temperature PEMs. Fundamentally, an electrochemical pump is designed to oxidize and reduce hydrogen at the anode and cathode, respectively in an electrolytic mode. The concept is simple, requires little power, and has been shown to pump hydrogen to high pressures. However, the original work based on low temperature PEMs was constrained by the same gas purity requirements that are imposed on today's low temperature PEMFCs, i.e., the hydrogen stream was not processable if it contained trace amount of CO and/or H2S. High temperature PBI-based electrochemical hydrogen pumping has been developed by H2PUMP, LLC.
As such, a need exists for a method of operating a high temperature PBI-based electrochemical hydrogen pump to purify a hydrogen gas stream with high concentration of hydrogen sulfide.